Paper products are well known in everyday life. Paper products are frequently referred to as tissue, and are used for paper toweling, facial tissue, and bath tissue.
Tissue paper products may comprise a single ply, but frequently comprise two or more plies. As used herein, a "ply" refers to a single sheet taken off a forming wire, or the equivalent thereof, and dried without additional fibers being added thereto.
Of course, a ply may be layered with different cellulosic fibers. Layering provides the benefits that a central layer may comprise relatively strong fibers to impart strength to the tissue paper product. Outboard of the central layer may be shorter fibers which impart a soft tactile sensation to the user. Layering may be advantageously accomplished by commonly assigned U.S. Pat. No. 3,994,771, issued Nov. 30, 1976 to Morgan, Jr. et al., which patent is incorporated herein by reference.
Frequently, two or more plies are joined together to make the paper product. Joining multiple plies together provides the advantage that the resulting laminate has a lesser bending modulus than a single ply of equivalent thickness. This provides the benefit that, again, a softer tactile sensation is perceived by the user. Absorbency and caliper are typically improved as well. Furthermore, joining three plies together allows the paper product to have different central and outboard plies in the laminate, to provide strength and softness respectively.
Multi-ply tissue products are typically cellulosic. As used herein, "cellulosic" refers to a paper product comprising at least about fifty weight percent or at least about fifty volume percent cellulosic fibers including, but not limited to, cotton linters, rayon, bagasse, and more preferably wood pulps, such as softwoods (gymnosperms or coniferous) or hardwoods (angiosperms or deciduous), which fibers may be recycled. The balance of the fibers may be synthetic, such as polyolefin or polyester.
Cellulosic plies are frequently joined together by the use of an adhesive. Adhesive joining of cellulosic plies is advantageously described in commonly assigned U.S. Pat. No. 5,143,776, issued Sep. 1, 1992 to Givens, which patent is incorporated herein by reference.
However, adhesive joining of multiple cellulosic plies in a paper product can, and has, led to unsatisfactory performance. Particularly, paper products used as paper toweling, facial tissue, and bath tissue must have the proper ply bond strength. As used herein, "ply bond strength" refers to the force necessary to separate two adjacent plies from one another as described below.
Frequently tissue paper products, particularly paper toweling, are wetted in use. If the wet ply bond strength is insufficient, the plies separate in use and the paper product is destroyed. While it would seem an easy matter to simply increase the wet ply bond strength, the dry ply bond strength is directly coupled to the wet ply bond strength. In the prior art, as the wet ply bond strength increases to the proper level, the dry ply bond strength becomes too great. When the dry ply bond strength is too great, the softness and absorbency are typically reduced.
Accordingly it is an object of this invention to provide a multi-ply paper product. It is further an object of this invention to provide a multi-ply paper towel product having adequate wet ply bond strength without having a dry ply bond strength which is too great. Finally it is an object of this invention to provide such a paper product which is cellulosic and has the plies adhesively joined.